This invention relates to modulating optical signals in waveguides. More particularly this invention relates to an integrated Mach-Zehnder Interferometer (MZI) design which employs a particular optical coupler at the point where the MZI arms recombine that steers a light beam into a central output waveguide if the beams entering the coupler are in phase and steers the light which is not in phase into two diverging output waveguides that flank the central output waveguide. Phase modulators in at least one arm of the MZI control the phase matching in the coupler and the output of the central output waveguide.
Compact, low-cost photonic integrated circuits (PICs) have long been a desire of systems engineers. Unfortunately, the majority of PICs in use today use regrown buried heterostructure waveguides to achieve low crosstalk at reasonable packing density. These regrown structures are very expensive and limit PIC applications to high performance niches. The alternative low-cost approach is to use etched-rib, or strip-loaded, waveguides. Strip-loaded waveguides are simple to manufacture but may have guided slab-modes carrying unwanted light between devices within the PIC. These slab modes can result in very high crosstalk or low device density. In addition, some devices such as Mach-Zehnder interferometers and waveguide power couplers generate radiation into slab modes as a fundamental means of operation. These strip-loaded waveguide PICs have been of limited utility due to the exceedingly high crosstalk between optical elements. This crosstalk results from the existence of slab-type optical waveguide modes outside the actual strip waveguide. Attempts to eliminate crosstalk by etching away the unwanted slab waveguide material results in highly overmoded mesa isolated waveguides. These mesa isolated structures can be useful if care is taken not to couple light into the higher-order modes. Nevertheless, problems arise when using a standard y-junction waveguide power coupler with this sort of structure since this device operates by coupling all out-of-phase light into radiation modes so that it can leave the optical circuit. When using multi-mode mesa isolated strip-loaded waveguides, the y-junction also couples light very strongly into the higher-order modes of the mesa structure. This light is then free to propagate along the mesa such that it is difficult to distinguish from light in the guided rib mode. Light guided by the mesa may also couple, by means of scattering, back into the strip waveguide. Both phenomena cause a dramatic reduction in contrast ratio or modulation depth.
An example of a conventional etched-rib waveguide implementation of an MZI with a standard Y-coupler is shown is FIG. 1. Light enters the MZI section 10 from an input rib waveguide 18 and splits along the two arms 13 and 15, passing through the two phase shifters 16 into the Y-coupler 14. The light entering the Y-coupler 14 will either preferentially couple into the output waveguide 20 if the light coming out of the two arms 13 and 15 is in phase or will couple, if out of phase, into the slab as higher order modes or will radiate as shown by the arrows 22. This radiated light is forced to couple into either substrate modes, air modes, or one-dimensionally guided slab-waveguide modes supported by high-refractive-index material remaining at the upper surface of the semiconductor after the etched rib is formed. The bulk of the light radiated out of the rib waveguide couples into the slab waveguide, if present, allowing for crosstalk between adjacent waveguides. Thus, the out of phase light component is responsible for the undesirable cross talk between other devices on a PIC when conventional Y couplers are used. Furthermore, light coupled into the slab waveguide by a conventional Y coupler can reduce the MZI contrast ratio (on-off ratio) if it does not propogate away from the output rib waveguide 20 sufficiently and therefor is allowed to partially couple into other optical elements of the PIC (such as an an optical fiber, detector or cascaded waveguide switch).